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United States Patent |
6,008,488
|
Jarvis
,   et al.
|
December 28, 1999
|
Rayleigh backscatter control apparatus and method
Abstract
An analyzer for analyzing chemical properties of a specimen using optical
coupling and sensing, has a narrow band light source and an optical probe
that is in contact with the specimen. Detectors are used for providing
electrical outputs as a function of a parameter of the specimen, and an
optical path couples light from the light source to the probe and also
couples Raman scattered light from the specimen and Rayleigh scattered
light from the specimen to separate detectors. An analyzer circuit is
coupled to the detectors to provide outputs as a function of the
electrical signals received, and one of the detectors is connected to a
signal conditioning circuit for controlling the input of the light source
and includes an asymmetric discrimination circuit configured to
distinguish positive going fluctuations from negative going fluctuations
in the output from the detector.
Inventors:
|
Jarvis; John M. (Bloomington, MN);
O'Donnell; John J. (Ennis, IE);
O'Connor; Eamon (Limerick, IE)
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Assignee:
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Rosemount Analytical Inc. (Orrville, OH)
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Appl. No.:
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038348 |
Filed:
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March 11, 1998 |
Current U.S. Class: |
250/227.23; 250/574 |
Intern'l Class: |
H01J 005/16 |
Field of Search: |
250/227.23,227.24,574,573,576
356/301-303
422/68.1,79
436/43,46
|
References Cited
U.S. Patent Documents
3985508 | Oct., 1976 | Williams | 422/65.
|
4127329 | Nov., 1978 | Chang et al. | 356/301.
|
4648714 | Mar., 1987 | Benner et al. | 356/301.
|
4953976 | Sep., 1990 | Adler-Golden et al. | 356/301.
|
5112127 | May., 1992 | Carrabba et al. | 356/301.
|
5455673 | Oct., 1995 | Alsmeyer et al. | 356/301.
|
5678751 | Oct., 1997 | Buchanan et al. | 228/124.
|
Foreign Patent Documents |
0 405 752 A2 | Jan., 1991 | EP.
| |
Other References
"Riber Raman background study and its application in setting up optical
fiber Raman probes", by Jiaying Ma and Ying-Sing Li, Applied Optics, May
20, 1996, vol. 35, No. 15, pp. 2527-2533.
|
Primary Examiner: Le; Que T.
Attorney, Agent or Firm: Westman, Champlin & Kelly, P.A.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is related to the following copending applications which
are incorporated herein by reference and filed on even date herewith:
"Chemical Analyzer Optical Probe and Method of Manufacturing Same" (Ser.
No. 09/038,350, filed on Mar. 11, 1998), "Improved Low Noise Raman
Analyzer System" (Ser. No. 09/038,438, filed on Mar. 11, 1998), and
"Chemical Analyzer With Free Space Communication Link" (Ser. No.
09/038,443, filed on Mar. 11, 1998), all having common ownership and
inventorship with the present application; and "Raleigh Backscatter
Control Apparatus and Method" (Ser. No. 08/947,816, filed Oct. 9, 1997;
attorney docket No.119271720), and "Method For Standardizing Raman
Spectrometers To Obtain Stable And Transferable Calibrations" (Ser. No.
08/947,689, filed on Oct. 9, 1997now U.S. Pat. No. 5,850,623; attorney
docket No.11927/710), assigned to Eastman Chemical Co. This application is
based upon a provisional application Ser. No. 60/039,084, filed on Mar.
14, 1997.
Claims
What is claimed is:
1. In an instrument for analyzing a specimen of interest, the instrument
having at least:
a narrow-band light source having a control input;
an optical probe having a terminus to contact the specimen;
a first and second detector having respectively a first and second
electrical output;
an optical path coupling light from the light source to the probe terminus,
coupling Raman-scattered light from the specimen to the first detector,
and coupling Rayleigh-scattered light from the specimen to the second
detector;
an analyzer circuit coupled to the first detector and providing an analyzer
output as a function of the first electrical output; and
a signal conditioning circuit coupled to the second detector and
controlling the control input of the light source as a function of the
second electrical output; the improvement comprising:
an asymmetric discrimination circuit, included in the signal conditioning
circuit, configured to distinguish positive-going fluctuations from
negative-going fluctuations in the second electrical output.
2. The improvement of claim 1, wherein the asymmetric discrimination
circuit blocks transient decreases but passes transient increases in the
second electrical output.
3. The improvement of claim 1, wherein the signal conditioning circuit
further includes a pulse generator coupled to the control input.
4. The improvement of claim 3, wherein the signal conditioning circuit
further includes an OR gate coupling the pulse generator and the
asymmetric discrimination circuit to the control input.
5. The improvement of claim 1, wherein the optical path includes a bandpass
filter, interposed between the specimen and the second detector, that
transmits the Rayleigh-scattered light.
6. In a method for analyzing a specimen of interest with an optical probe,
including:
providing a narrow-band light source;
providing an optical path carrying light from the light source to the
probe; method for analyzing a specimen of interest with an optical probe,
providing a control signal to control the light source, the control signal
having a first state corresponding to a high light level sufficient to
produce detectable Raman- and Rayleigh-scattered light signals from the
specimen and having a second state corresponding to a lower light level;
providing an analyzer output as a function of the Raman-scattered light
signal; and
comparing the Rayleigh-scattered light signal to a threshold value and
forcing the control signal to the first state if the Rayleigh-scattered
light signal is above the threshold value;
the improvement comprising:
pulsing the control signal to the first state if the Rayleigh-scattered
light signal is below the threshold value.
7. The improved method of claim 6, wherein the pulsing step is repeated
periodically.
8. The improved method of claim 6, wherein the pulsing step has associated
therewith a pulse width of not more than 10 msec.
9. The improved method of claim 8, wherein the pulse width is about 5 msec.
Description
BACKGROUND OF THE INVENTION
The present invention relates to instruments that analyze chemical
properties of a specimen using optical means. More specifically, the
invention is an improvement to already developed techniques for
controlling a laser or other narrow-band light source in such instruments.
BRIEF SUMMARY OF THE INVENTION
A first aspect of the present invention relates to the improvement of
introducing a pulse generator in a signal conditioning circuit of a
chemical analyzer that analyzes a specimen. The analyzer has a narrow-band
light source with a control input, an optical probe, and a first and
second detector having a first and second detector output respectively.
The analyzer also has an optical path coupling light from the light source
to the probe, coupling Raman-scattered light from the specimen to the
first detector, and coupling Rayleigh-scattered light from the specimen to
the second detector. An analyzer circuit couples to the first detector,
and provides an analyzer output as a function of the first detector
output. The analyzer also includes a signal conditioning circuit coupled
to the second detector and controlling the control input as a function of
the second detector output. The pulse generator momentarily triggers the
light source via the control input to test for the presence of the
Rayleigh scattered light on startup or after a lapse of the
Rayleigh-scattered light, without controlling the light source to
unnecessarily high levels.
Another aspect of the invention relates to the improvement of introducing
an asymmetric discrimination circuit in the signal conditioning circuit of
the described analyzer. The discrimination circuit distinguishes
positive-going fluctuations from negative-going fluctuations of the second
detector output, causing the signal conditioning circuit to block
transient decreases but pass transient increases in the second detector
output. when used in combination with the pulse generator, the asymmetric
discrimination circuit can permit the signal conditioning circuit to
rapidly respond to a transient increase in detected Raleigh-scattered
light, while preventing the signal conditioning circuit from rapidly
responding to a transient decrease in detected Raleigh-scattered light.
According to another aspect of the invention, a bandpass filter is
interposed between the specimen and the second detector, making the
analyzer less sensitive to extraneous light entering the optical path.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a system diagram of a preferred chemical analyzer in
accordance with an aspect of the invention;
FIG. 2 depicts chief components of a preferred optical filter used in the
analyzer of FIG. 1;
FIG. 3 is an enlarged end view of optical fibers in the probe shown in
FIGS. 1 and 5;
FIG. 4 is a perspective end view of a male fiber connector showing an
arrangement of optical fibers at the connector end as used in the analyzer
of FIG. 1;
FIG. 5 is a partially sectional view of a preferred optical probe useable
with the analyzer of FIG. 1;
FIG. 6 is a sectional view of a distal end of the probe of FIG. 5 during
probe fabrication;
FIG. 7 is a block schematic of a preferred signal conditioning circuit
useable with the analyzer of FIG. 1;
FIG. 8 is a schematic of a comparator/asymmetric discriminator circuit
depicted as a block in FIG. 7; and
FIG. 9 is a timeline showing signals at different points in the signal
conditioning circuit of FIG. 7.
For convenience, items in the figures having the same reference symbol are
the same or serve the same or a similar function.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
As used herein, "optic" (al) and "light" refer to electromagnetic
radiation, whether or not visible to the human eye.
FIG. 1 shows a preferred analyzer 10 that evaluates a specimen of interest
12 and provides on a computer 14 or other suitable output medium an
analyzer output indicative of the presence or amount of one or more
chemical constituents of the specimen. The analyzer 10 illuminates the
specimen 12 with narrow-band light, collects backscattered light from the
specimen, optically isolates a Raman scattering component from the
backscattered light, and evaluates the Raman scattering component to
calculate the analyzer output. Simultaneously, the analyzer isolates a
Rayleigh scattering component from the backscattered light. If the
Rayleigh scattering component falls below a threshold level, which may
result for example from disconnecting a fiber connector or withdrawing the
probe from the specimen, the narrow-band light illumination is shut off.
This shut off technique is most effective in specimens such as liquids
that have significantly higher Rayleigh scattering levels than that of
gasses such as air.
A diode laser 16 launches essentially monochromatic, narrow-band light into
a fiber 18a of a 2-by-2 beamsplitter 20. A wavelength of about 810
nanometers (nm) for the narrow-band light has been found satisfactory.
Shorter wavelengths increase the amount of Raman scattering, but may also
produce unwanted fluorescence in some specimens; longer wavelengths are
less likely to produce fluorescence but yield a lower Raman signal. In
practice, a diode laser having a wavelength between about 750 and 850 nm
is preferred. The selected wavelength should not however coincide with an
absorption line of the specimen 12, if maximum Raman scattering levels are
desired.
The laser can have a multimode output and be capable of emitting 700 mW to
1.2 W of optical power during analyzer operation. Laser 16 also includes a
driver circuit with a control input at line 24, and a temperature control
circuit if the laser source is a diode laser. The control input of laser
16 controls the amount or intensity of narrow band light injected into
fiber 18a.
Beamsplitter 20 divides the laser light launched into fiber 18a between
fibers 18b, 22a, preferably in equal amounts although other ratios are
also contemplated. The narrow band light passes from fiber 18b to a fiber
18c via a connector pair 26. Connector pair 26 includes a male connector
end holding each fiber end, the connector ends facing each other inside an
alignment bushing. SM A-type connector pairs are preferred for robustness
and ease of modification to nonstandard bore sizes, but other known styles
such as ST or FC are also contemplated. Fiber 18c connects to a bandpass
filter 28 in a fiber termination/filter module 30. Laser light passes
through filter 28 to a fiber 18d, which carries the light to a probe 32
adapted to contact the specimen 12. Fibers 18c, 18d are preferably pan of
armored cable assemblies 34,36 respectively.
The analyzer 10 is preferably arranged as a main analyzer unit 38 situated
in a 10 control room (or other suitable location that can provide the
necessary electrical power), a probe 32 located at the specimen, and the
termination/filter module 30 located near the probe. Main analyzer unit 38
is preferably housed in an intrinsically safe enclosure, configured with
an industry standard Z-purge capability at a port 38a that maintains a
positive air pressure inside unit 38 relative to its surroundings. Armored
fiber cable assemblies 34,36 connect unit 38 to module 30, and module 30
to probe 32, respectively. Cable assembly 34 can be tens or hundreds of
meters long.
Optical fiber can itself generate Raman scattering and/or fluorescence
(hereinafter, "spurious light signals") from laser light passing through
it, which if detected can be confused with Raman scattering from the
specimen. The spurious light signals are a function of the fiber
properties (most importance its length, but also including core material,
cladding material, and buffer layer material), and generally have
wavelengths longer than the laser wavelength. Therefore, bandpass filter
28 is provided in the termination/filter module 30, and module 30 is
located as close to the probe as possible to minimize fiber 18d length and
thus minimize any spurious light signals generated in fiber 18d. In a
benign environment, the cable assembly 36 (including fiber 18d) can be
eliminated and the filter 28 and the other filter (74, discussed below)
can be mounted directly on the proximal end of the probe 32. However, in
many practical applications the temperature or change in temperature
encountered even at the proximal end of the probe can have adverse effects
on filter performance. Hence, module 30 mounted away from probe 30
provides a more stable temperature environment for the filters 28,74.
Bandpass filter 28 blocks the spurious light signals originating in fibers
18a,18b,18c, from reaching fiber 18d, but passes narrow band light from
laser 16.
Spurious light signals can be further reduced by using silica-based fiber
having an inert metal buffer layer such as gold. Such fibers have
significantly lower spurious light signals than similar fibers having a
polymer-based buffer layer such as polyamide. The relatively high cost of
the metal-coated fibers however can make it impractical to use them
exclusively in analyzer 10, depending on the distances involved. Instead,
the use of filters 28,74 permits lower cost, polymer-based fibers to be
used between the main analyzer unit 38 and the filters, and the preferred
gold-coated fibers to be used in probe 32 and between probe 32 and the
filters.
A preferred embodiment of filter 28 is depicted in FIG. 2. Male fiber
connectors 39a,39b hold the ends of fibers 18c,18d against 0.25-pitch
gradient index (GRIN) lenses 40a,40b respectively. A filter 42 sandwiched
between lenses 40a,40b provides the desired spectral filtering
characteristics. Filter 42 preferably comprises an interference-type
filter.
Turning again to FIG. 1, fiber 18d releasably connects to probe 32 by a
connector pair 44 located at a proximal end 32a of probe 32 and passes
narrow band light to a fiber 18e that extends from connector pair 44 to a
distal end 32b of probe 32. Also housed in probe 32 are a fiber 46a and a
group of five fibers 45a. The line representing fibers 48a, and some other
lines in FIG. 1, are shown thickened to indicate multiple optical fibers
in the preferred embodiment. Fiber 46a connects to a connector pair 50 at
proximal end 32a, and at distal end 32b is brazed or otherwise held in
position together with fibers 18e and 45a. At end 32b, the fibers are
polished to a mirror finish and arranged as shown in FIG. 3. A braze
material 52 holds the fiber ends in place and hermetically seals the probe
at end 32b. The probe design and construction is discussed further below.
Narrow band light exits fiber 18e and illuminates specimen 12 in a
detection zone 54 defined by the numerical aperture of fiber 18e. The
surrounding six fibers (46a and 45a) collect some of the backscattered
light, which will include a relatively strong Rayleigh component (same
wavelength as the narrow band light) and a relatively weak Raman component
(longer wavelength than the narrow band light). As will be seen, the
preferred analyzer 10 uses the multiple fibers 45a to boost the detected
Raman component and the single fiber 46a to sense the Rayleigh component
for continuity.
Backscattered light traveling down fiber 46a is directed to a detector 56
via fibers 46b-e, connector pairs 58,60, and a bandpass filter 62. Filter
62 passes the narrow band light wavelength and is substantially identical
to previously described filter 28. The purpose of filter 62 is to prevent
sunlight, room light, or any other extraneous light collected by fiber 46a
from being mistaken for Rayleigh backscattered light. Filter 62 also has
the effect of preventing the weaker Raman component, if present, from
reaching detector 56. An amplifier 64 couples to detector 56 to provide an
amplified detector output on line 66.
Advantageously, the detector 56 output, representative of the Rayleigh
scattering component, is fed back through a signal conditioning circuit 68
to the laser control input at line 24. Circuit 68 compares the detector
output on line 66 with a predefined threshold. If the detector output is
above the threshold, indicating that the analyzer optical system is
intact, circuit 68 provides an output on line 24 that maintains laser 16
at its normal, relatively high, output level. If on the other hand the
detector output is below the threshold, indicating a fiber disconnection
or break, or removal of the probe from the specimen, circuit 68 provides
an output on line 24 that shuts off laser 16 or at least controls it to a
lower intensity level. This lower intensity level can be set such that the
light intensity emitted from probe fiber 18e, and preferably from fiber
18b, is within BSI/EN 60825-1 class 1 operation (i.e., nonhazardous direct
viewing). In this way, analyzer 10 can operate with high laser light
levels during normal operation and automatically shut down if a
discontinuity is sensed by detector 56, thereby avoiding hazardous viewing
by an operator.
Several enhancements to the basic shut-down technique are also provided by
signal conditioning circuit 68. One enhancement is the ability to
discriminate between transient losses in the Rayleigh scattering
component, such as may be caused by small bubbles 69 of air or other gas
passing through the detection zone 54, and longer lived losses in the
signal which may result from fiber disconnection or break, or a withdrawal
of probe 32 from the specimen. The circuit 68 continues driving laser 16
at its high operational intensity level in the presence of the truly
transient losses, but shuts the laser down to the lower intensity level
for the longer-lived losses. This discrimination function prevents
unnecessary and annoying shutdowns during operation of analyzer 10.
Another enhancement involves periodically interrogating the laser 16 after
a shutdown has occurred, or at startup, so that if system integrity is
restored the analyzer 10 will automatically return to normal operation
(i.e., high laser intensity level). These capabilities of circuit 68 are
described in more detail below.
Turning again to probe 32 in FIG. 1, fibers 48a are unsupported in probe 32
except at distal end 32b, where they are arranged around the emitting
fiber as shown in FIG. 3, and at the proximal end 32a, where they are
bundled together at a connector pair 70. One of the male connector ends
70a of pair 70 holds the five fibers 48a as shown in the perspective end
view of FIG. 4. The other male connector end of pair 70 holds a single
fiber 48b in alignment with the fibers 48a, where fiber 48b has a diameter
sufficiently large to capture light emitted from all of the fibers 48a.
For example, if fibers 48 are 100 .mu.m (core diameter), fiber 48b can be
about 300 .mu.m (core dia.). This arrangement greatly simplifies analyzer
10 interconnections: rather than five separate fibers, connector pairs,
and filters connecting the Raman pickup fiber channel from the probe 32 to
the main analyzer unit 38, only one-fifth of those components are required
by using a large fiber to collect light from fibers 48.
Backscattered light is carried by fiber 48b to the entrance slit of an
optical spectrograph 72 via a long pass filter 74, fibers 48c-e, and
connector pairs 76,78. Long pass filter 74 has the same construction as
the bandpass filter shown in FIG. 2 except that the filter element 42 is
fabricated to block the narrow band light of laser 16 and pass longer
wavelengths. Preferably the spectral transmission of filter 74 is less
than 10.sup.-6 at the laser wavelength 810 nm and rises to half of its
peak transmittance (70% typ. peak trans.) at about 833 nm. As discussed,
fiber termination/filter module 30 is mounted close to probe 32 to keep
fiber 48b short (generally no more than a few, and preferably <1 meter),
so that no spurious light signals can be produced by Rayleigh
backscattered light in fiber 48b. Filter 74 blocks any Rayleigh
backscattered light from reaching fibers 48c-e. Fiber 48b preferably has a
metal (gold)-buffer layer.
Fibers 18a-e, 46a, 48a, 48e, and 22a-d are preferably relatively small
diameter (e.g. 10 .mu.m core) fibers, while fibers 48b-d are preferably
relatively large diameter (e.g. 300 .mu.m core) fibers. Fibers 46b-e can
be either small or large diameter, but preferably are no smaller than
fiber 46a. All can be graded-index or, preferably, step-index for
increased light levels. Fibers 48e are held at connector pair 78 in a
substantially circular pattern (similar to FIG. 4) for optimal coupling to
fiber 48d, while at the entrance slit to spectrograph 72 they are held in
a linear array. Fibers 18d, 18e, 46a, 48a, and 48b all have inert metal
buffer layers, preferably gold.
A diamond reference 80 is provided in main analyzer unit 38. Fibers 18a,
22a,22b carry narrow band light from laser 16 to the surface of diamond
80. Bandpass filter 82, substantially identical to filters 28 and 62,
blocks fiber-generated Raman scattering. Six fibers 22c surround fiber 22b
at the diamond surface (similar to FIG. 3) to capture backscattered light
from diamond 80. A longpass filter 84, substantially identical to filter
74, blocks Rayleigh scattered light from fibers 22d. Fibers 22d, six in
number, are arranged circularly at filter 84 and linearly at the
spectrograph 72 entrance slit.
The linear arrays of fibers 22d and 48e are arranged colinearly, one
abutting the other, at the entrance slit to spectrograph 72. Spectrograph
72 is preferably equivalent to model SP-150 available from Acton Research
Corp., and has a ruled grating with 400 grooves/mm and blazed at 750 nm. A
detector array 74, preferably 750 pixels wide by 240 pixels high,
intercepts and simultaneously monitors the spatially separated Raman
scattered light spectra from the specimen 12 and from the diamond
reference 80. The output from detector array 74 is fed to computer 14 over
a line 75. Signal processing software residing in computer 14 is used to
produce a standardized Raman spectrum of the specimen (see copending
application of Eastman Chemical Co. entitled "Method For Standardizing
Raman Spectrometers To Obtain Stable And Transferable Calibrations",
attorney docket number 11927/710 and filed on Oct. 9, 1997, incorporated
herein by reference) using the diamond Raman spectrum. Pattern recognition
software also residing in computer 14 calculates the chemical composition
of specimen 12 from the standardized Raman spectrum of the specimen and
calibration training data. Such pattern recognition software is available
from Galactic Industries, Boston, Mass.
As previously mentioned, main analyzer unit 38 is preferably intrinsically
safe. Although computers having intrinsically safe keyboards and monitors
are commercially available, there are significant difficulties in
providing a convenient and aesthetic user interface using these
components. Therefore, computer 14 is preferably equipped with a
transceiver 86 such as an antenna or an infrared transmitter/receiver. A
user can send instructions to and receive information from computer 14
using a second device such as a laptop computer 88 equipped with a similar
transceiver 90. Such communication preferably occurs over a wireless,
fiberless free space path 91, allowing the user to freely move from place
to place with computer 88 and permitting greater flexibility and choice in
a mounting location for main analyzer unit 38. Preferred transceivers
86,90 are commercially available radio LAN cards for desktop or laptop
computers, for example the WAVELAN card available from AT&T Lucent
Technologies, designed to interface to a standard PC/MCIA slot or Industry
Standard Architecture (ISA) bus slot. Transceiver 86 is depicted in FIG. 1
as such an antenna device, that partially extends out of the housing of
unit 38 and connects to computer 14 by a coax line 86a. The transceiver
can also be an infrared emitter/receiver disposed inside the housing of
unit 38 behind a window. Computer 88 has a keyboard and a mouse that are
used to send queries and commands to computer 14. Computer 88 also has a
display to graph or otherwise show the analyzer output data transmitted
from computer 14. With this arrangement, computer 14 is preferably
equipped with neither a display screen, a keyboard, nor a mouse, to
satisfy intrinsic safety requirements as well as to reduce the size,
weight, and electrical requirements of main analyzer unit 38.
Probe 32 is described in more detail in connection with FIGS. 5 and 6,
together with previously discussed FIGS. 1, 3, and 4. Probe 32 preferably
has a probe body with three main components: a terminus 32c, a shank 32d,
and a connector housing 32e, all made of 316 stainless steel or other
suitable inert materials capable of withstanding several hundred degree C
temperatures, such as Hastelloy C. Terminus 32c, shank 32d, and housing
32e are rotationally symmetric about a probe axis 32f, and are connected
by braze joints 92,94 as shown. The outer diameter of terminus 32c, braze
joint 92, and shank 32d is polished to a smooth finish (0.25 in. dia.) to
permit sealing with a ferrule inside the bore of a standard pipe fitting,
such as those available from Swagelok Corp., or the bore of some other
container that holds specimen 12.
Fibers 18e, 46a, and 48a (only one of which is shown in its entirety in
FIGS. 5 and 6 for simplicity) extend from their respective male connector
ends 44a, 50a, and 70a at proximal probe end 32a to distal end 32b. Each
of the fibers are step-index, with silica or doped silica core/cladding,
and have a thin outer buffer layer of gold, nickel, or other inert metal
along their entire length. Male connector ends 50a, 44a, and 70a are fixed
to connector housing 32e to permit probe 32 to be conveniently
disconnected and reconnected to cable assembly 36 for ease of installation
and servicing. Also affixed to housing 32e is a connector 96 for a
temperature sensor 98 included in probe 32. Temperature sensor 98 is
preferably disposed proximate distal end 32b for diagnostic purposes to
ensure that probe 32 does not exceed its rated temperature. Alternately,
the output of sensor 98 can be used as a rough indication of the specimen
temperature, whereupon probe 32 takes on a dual role as a fiber optic
chemical analysis probe and a specimen thermometer. Although known fiber
optic temperature sensors can be used for sensor 98, electrical sensors
are preferable for their simplicity, and most preferable is a thermocouple
(e.g. type K) for its low-cost and reliability. The output of sensor 98
can be monitored with a portable, hand-held device coupled directly to
connector 96, or with computer 14, in which case an additional channel
such as a twisted wire pair can be included in cable assemblies 36,34.
The procedure for brazing fibers 46a, 18e,48a into the stainless steel
terminus 32c will now be described. To enhance adhesion, terminus 32c is
plated with gold 100 or other metal matching the metal buffer layer of the
fibers. The gold plating extends inside a bore 104 and in the vicinity
thereof, but preferably is removed from or not provided on the remaining
surfaces of terminus 32c. This is to keep molten braze material in the
vicinity of bore 104 during fabrication, preventing it from spreading over
the entire terminus 32c. An anti-wicking agent or stop-flow substance,
preferably a suspension of magnesium hydroxide in water, is applied to
each of the fibers in a zone indicated generally at 106 prior to brazing.
Zone 106 approaches but does not touch the distal portion of the fibers
that extend into bore 104. The anti-wicking agent inhibits the flow of
molten braze material along the fibers substantially beyond bore 104. With
the fibers and terminus 32c so prepared, the assembly is positioned in a
vacuum oven 108 as shown in FIG. 6, with a small ring or loop of solid
braze material 110 resting on top of terminus 32c at or near bore 104.
Preferred braze materials for platings 100 made of gold are cadmium-free
varieties; widely available braze type (AWS) BAg-8, a binary alloy
composed of about 72% silver and 28% copper, is most preferable. The
vacuum oven 108 is then heated to a temperature sufficient to melt braze
material 110. By preparing the fibers and terminus 32 as described, the
molten braze material does not run out of but rather tends to stay in and
around the vicinity of bore 104, wicking between the fibers and filling
the spaces between them. Upon cooling, the braze material forms a solid
hermetic seal within bore 104, uniformly filling the inter-fibral spaces
inside bore 104 with few or no voids (see FIG. 3).
As final fabrication steps, a sleeve 112 of terminus 32c is brazed to shank
32d using localized heating, and shank 32d is then brazed to connector
housing 32e also using localized heating. The sleeve 112 partially
isolates the brazed fibers in bore 104 from heat generated during brazing
of terminus 32c to shank 32d. Braze material BAg-8 is used for all braze
joints. The fiber ends are polished to a flat, mirror-smooth finish at
distal end 32b. Lastly, the other fiber ends are potted into the male
connector ends, which connector ends are also brazed to connector housing
32e at proximal end 32a.
FIG. 7 depicts in block schematic form a preferred signal conditioning
circuit 68. Circuit 68 receives on line 66 the amplified detector output,
representative of the Rayleigh scattering component from specimen 12, and
provides on line 24 an output that controls a light output level of laser
16. A comparator/asymmetric discriminator circuit 114 compares the
amplified detector output to an adjustable internal threshold. The
threshold is adjusted according to the desired laser operational output
level, fiber attenuation losses, filter and connector pair losses, and
specimen scattering characteristics, to a level less than an output level
on line 66 for a fully intact system with the probe contacting the
specimen, and greater than a lower output level corresponding to the
amount of Rayleigh scattered light received when the probe is withdrawn
from the specimen and pointed into the air, or when one of the fiber
connector pairs is uncoupled. The output of circuit 114 feeds into an OR
gate 116 and a latch 118 as shown. OR gate 116 in turn drives a FET
transistor 120 which connects directly or through one or more buffer
amplifiers if desired to line 24. Thus, as long as analyzer 10 is intact
and probe 32 is disposed in the specimen, the signal on line 66 will be
higher than the threshold level, the output of circuit 114 will be "HI",
the output of OR gate 116 will be "HI", turning transistor 120 "ON" to
couple the +12V voltage to line 24, thereby maintaining laser 16 at its
high operational output intensity. If the probe 32 is withdrawn from the
specimen, however, the output of circuit 114 will go "LO", as will OR gate
116, turning off transistor 120 and forcing laser 16 to a lower
(preferably zero) intensity level.
Circuit 114 also preferably performs a discrimination function against
transient losses of the detected Rayleigh scattering component. This
function is described in connection with FIG. 8.
The latch 118 is provided so that computer 14 can monitor the activity of
circuit 114. An output line 118a conveys the status of the latch to the
computer, and a reset line 118b permits the computer to reset the latch.
Advantageously, circuit 68 also includes a low duty cycle pulse generator
122 that also feeds into OR gate 116. In a preferred embodiment, a pulse
having a 5 millisecond (ms) duration is generated at a 1 Hz repetition
rate. When the laser 16 is in its zero or low intensity state, as it would
be on power-up of analyzer 10 and as it would be after a drop in the
detected Rayleigh scattering component below the threshold, each pulse
from generator 122 causes the laser to momentarily (for the duration of
the pulse) provide the higher output intensity. The pulses are kept short
enough, and the duty cycle small enough, to keep the light emitted from
probe 32 or 5 even from connector pair 22 below the safety limits for the
human eye and for explosive atmosphere environments. When system integrity
returns to analyzer 10, the Rayleigh backscatter signal will return to
line 66 during one of the pulses, causing circuit 114 to turn "ON",
thereby establishing normal analyzer operation.
Still another input to OR gate 116 is a manual override pushbutton 124.
When activated, pushbutton 124 forces laser 16 to the high output
intensity. This capability is provided for troubleshooting purposes.
Turning now to FIG. 8, the asymmetric transient discrimination feature of
comparator/asymmetric discriminator circuit 114 will be described. The
amplifier 64 is shown in more detail as a first stage transimpedance
amplifier and a second stage amplifier with gain. The circuit 114 is shown
as three circuits 114a, 114b, 114c connected in series. Circuit 114a,
configured as shown, performs the comparator function described
previously. Adjustment of potentiometer 126 adjusts the electric potential
at the noninverting input of operational amplifier 128, which electric
potential functions as the threshold referred to previously, against which
the amplified detector output on line 66 is compared. Since operational
amplifier 128 is wired as a comparator, it has essentially a digital
output. This digital output changes state very rapidly every time the
signal on line 66 crosses the threshold potential at the noninverting
input of op amp 128. Ignoring circuit 114b for the moment, if circuit 114a
was directly connected to circuit 114c, the circuit 114 output on line 130
would respond equally as rapidly to positive-going and negative-going
changes in the line 66 signal as it crossed the threshold. However, as is
apparent from the wiring of FET transistor 132 in circuit 114c, the
polarity of the signal on line 130 is opposite that of the signal at the
output of op amp 128.
Circuit 114b, however, discriminates between positive-going and
negative-going transitions. During normal analyzer operation, with a
Rayleigh scattering component above the threshold level, the output of op
amp 128 is LO, the potential at node 134 is LO, capacitor C1 is not
charged, and transistor 132 is off. If the Rayleigh scattering component
suddenly drops below the threshold level, the output of op amp 128
immediately goes HI. Diode D1 is reverse biased (nonconducting), and the
combination of resistors R1. R2, and capacitor C1 delay the turn-on of
transistor 132. The delay ("T") is proportional to (R1+R2)*C1. If the
Rayleigh scattering component stays below the threshold level for at least
the delay time T, transistor 132 will turn on, causing the laser 16 to
shut down. If however the detected Rayleigh component returns to a level
above the threshold level before time .iota. has elapsed, op amp 128
output will immediately go LO, diode D1 will be forward biased
(conducting), and capacitor C1 will discharge rapidly through only
resistor R2. Preferably, the value of R1 is much greater than R2. In a
preferred embodiment R1=200 k.OMEGA., R2=10 k.OMEGA., and C1=10
picofarads. Preferred delay times .iota. are in the range of about 0 to 44
ms, and are preferably programmable by computer 14 (e.g. by a
computer-controlled switch and one or more resistors in circuit 114b that
changes the effective resistance in parallel with diode D1).
In this manner, circuit 114b discriminates between a transitory loss in the
detected Rayleigh scattering component and a transitory appearance of such
component.
FIG. 9 depicts the output of pulse generator 122, the amplified detector
output on line 66, and the output of circuit 114 as waveforms 136, 138,
140 respectively. Broken line 142 represents the threshold level set in
circuit 114. At time t=0, the analyzer is powered up, the laser is off,
and the probe is withdrawn from the specimen. At times t.sub.1 and
t.sub.2, the pulse generator pulses the laser on, but only a very low
Rayleigh scattering component is detected since the probe is not
contacting the specimen. Waveform 140 therefore remains off. Between times
t.sub.2 and t.sub.3, the probe is inserted into the specimen, so that at
the next pulse of waveform 136 at time t.sub.3, a Rayleigh scattering
component above the threshold is produced in waveform 138, and circuit 114
(waveform 140) rapidly responds. Between time t.sub.4 and t.sub.5, small
bubbles passing through detection zone 54 cause transitory dips in
waveform 142 below threshold 142
The duration of such dips is less than .iota., 50 the waveform 140 remains
unchanged. Between time t.sub.5 and t.sub.6, a larger bubble passes
through detection zone 54, causing a dip in signal 138 with a duration
longer than .iota., whereupon waveform 140 drops to zero. By the time
t.sub.6, the large bubble has passed zone 54 and the pulse of waveform 136
brings back the Rayleigh scattering component in waveform 138 and the
output of circuit 114. A transitory increase in waveform 138 between
t.sub.6 and t.sub.7 has no effect on waveform 140, since waveform 138
stays above threshold 142 during that time. Between time t.sub.7 and
t.sub.85 an interruption such as a fiber break, fiber disconnection, or
probe withdrawal occurs. Waveform 138 responds immediately to the
interruption, while waveform 140 responds after the delay time .iota.
Although the present invention has been described with reference to
preferred S embodiments, workers skilled in the art will recognize that
changes may be made in form and detail without departing from the spirit
and scope of the invention. For example, analog circuits disclosed in the
preferred embodiment can be replaced with equivalent digital circuits,
e.g., DSP filters.
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